The Sat Nav is vastly more accurate than the speedo in the car. But only if you are doing a fairly constant speed on the level. It can get a little less accurate in stop start town traffic and up and down hills.
By law a car speedo can over read your speed by up to 10% + 4kph (it is an EU directive after all

). In other words if you are doing a true 30mph your speedo can legally read anything from 30 to about 34mph. But cannot tell you you are doing 28 for example.
The Sat Nav measures distance travelled and time far more accurately so is able to calculate your speed far better. But because it averages over a (short) time period it really only works properly if you are doing a fairly constant speed.
But it would not be advisable to rely on the sat nav. It can lose track of where you are and display some totally spurious speeds (my armchair at home is capable of 60+mph at times so my TomTom tells me

)
Use the speedo in the car and you will probably never actually exceed the speed limit if it is working within the legal tolerances. And you haven't done something like fit stupidly big wheels to the car!
And don't forget when it comes to getting a ticket it's not the speedo in your car or the TomTom you have to worry about. It's the one in that big white car with the blue lights on the roof behind you that will determine how many points you get....